CNC machine tool failure analysis and maintenance case summary

The application of CNC machine tools is more and more extensive, and its processing flexibility, high precision, high production efficiency, and many advantages. However, due to the increasingly advanced and complex technology, the quality of maintenance personnel is very high. They are required to have deep professional knowledge and rich maintenance experience, and can be eliminated in time when the CNC machine tool fails. The following is a systematic analysis of the faults of CNC machine tools with reference to some typical examples for reference.

First, the NC system failure

1. Hardware failure

Sometimes the machine is shut down due to hardware damage in the NC system. For the diagnosis of such faults, it is first necessary to understand the working principle of the numerical control system and the functions of each circuit board, and then analyze according to the fault phenomenon, and use the exchange method to accurately locate the fault point under certain conditions.

Example 1 , a CNC machine with Siemens SINUMERIK SYSTEM3 from Germany, its PLC adopts S5-130W/B, one fault occurs, the R parameter input through the NC system PC function does not work during processing, and can not change the R in the machining program. The value of the parameter. Through the analysis of the working principle and fault phenomenon of the NC system, we believe that the motherboard of the PLC has a problem, and after being replaced with the motherboard of another machine tool, it is further determined to be a problem of the PLC motherboard. After repair by professional manufacturers, the fault was eliminated.

Example 2 : The other machine tool also uses the SINUMERIK SYSTEM 3 numerical control system. The machining program program number input does not enter, and automatic machining cannot be performed. It was confirmed that there was a problem with the memory board of the NC system, and the fault was eliminated after the repair.

Example 3 : A CNC milling machine using the German HEIDENHAIN TNC 155, one failure, the system often crashes during work, and machine parameters and procedures are often lost during power failure. After inspection, it was found that the NC system main board was bent and deformed. After straightening and fixing, the system returned to normal and no similar fault occurred.

2. Soft fault

Some faults in CNC machine tools are caused by the machine parameters of the NC system. Sometimes, due to improper settings, sometimes the parameters are changed or confused due to accidents. Such faults will naturally disappear as long as the parameters are adjusted. There are also some faults that cause the NC system to be in an infinite loop due to accidental reasons. Sometimes such faults must be forced to start to restore the system.

Example 1 , a CNC lathe that uses the FANUC-OT system of Japan FANUC, crashes every time it is turned on, and any normal operation does not work. After the forced reset method is adopted, after the system memory is completely cleared, the system returns to normal, and after the machine parameters are re-entered, the machine tool is normally used. This fault is caused by chaotic machine parameters.

Example 2 : A special CNC milling machine, the NC system uses Siemens SINUMERIK SYSTEM 3, in the batch processing NC system displays the No. 2 alarm "LIMIT SWITCH", this fault is because the Y-axis stroke exceeds the limit set by the software, check The program value has not changed. After careful observation of the fault phenomenon, when the fault occurs, the Y-axis coordinate displayed on the CRT is determined to reach the software limit. Careful research is found that the compensation value input is large, and the software limit setting is properly adjusted. to be excluded. This fault is caused by improper software limit setting.

Example 3 : A CNC machine with Siemens SINUMERIK 810 has a problem at a time. Every time the system is turned on, it enters the AUTOMATIC state. No operation can be performed and the system crashes. After forced boot, the system resumes normal operation. This fault is caused by the operator's operation error or other reasons to make the NC system in an infinite loop state.

3. NC system failure due to other reasons may cause system failure due to problems with the power supply or buffer battery failure.

Example 1. A CNC machine tool using Siemens SINUMERIK SYSTEM 3 in Germany, once a fault occurs, after the NC system is powered up, the CRT is not displayed. It is found that the LED on the left side of the a€?COUPLING MODULEa€? board on the NC system is flashing, indicating fault. . After the hot start of the PLC, the system works normally. However, after a few days, this fault appeared again. After analyzing the flashing frequency of the LED, it was determined that the battery was faulty. After replacing the battery, the fault was eliminated.

Example 2 : A CNC machine tool using Siemens SINUMERIK 810, sometimes in the process of automatic machining, the system suddenly loses power, measuring its 24V DC power supply, and found that only 22V, when the grid voltage fluctuates downward, causing this voltage to drop, resulting in The NC system takes protective measures and automatically powers off. It was confirmed that the rectifier transformer was short-circuited between turns, resulting in insufficient capacity. After replacing the new rectifier transformer, the fault is eliminated.

Example 3 : The other is also a CNC machine tool using Siemens SINUMIK 810. When such a fault occurs, when the system is powered, the system starts self-test. When the self-test is completed, the system loses power. After analysis and inspection, it was found that the X-axis brake coil was short-circuited to the ground. After the system self-test, the servo conditions are ready and the brake is energized to release. The brake coil is powered by 24V power supply. Due to the short circuit of the coil to the ground, the voltage of 24V drops instantaneously, and the NC system takes protection measures to automatically cut off the power.

Second, the failure of the servo system

Since the control core of the numerical control system is to digitally control the feed part of the machine tool, the feed is controlled by the servo unit to drive the servo motor and drive the ball screw. The rotary encoder is used as the position feedback element to form the semi-closed loop position. Control System. Therefore, the role of the servo system on CNC machine tools is very important. The failure of the servo system is generally caused by problems such as the servo control unit, the servo motor, the speed measuring motor, and the encoder. Here are a few examples:

Example 1 : The servo motor is damaged.

A CNC lathe with SINUMERIK 810/T, one turret failure, the rotation is not in place, when the turret rotates, the alarm No. 6016 a€?SLIDE POWER PACK NO OPERATIONa€? appears, according to the working principle and fault phenomenon, the turret rotates. It is driven by the servo motor. When the motor starts, the servo unit generates an overload alarm, cuts off the servo power, and feeds back to the NC system to display the 6016 alarm. Checking the mechanical part and replacing the servo unit did not solve the problem. After replacing the servo motor, the fault is eliminated.

Example 2 : A US CNC grinding machine with DC servo system generates an a€?E AXIS EXECESSFOLLOWING ERRORa€? alarm during E-axis movement to observe the fault occurrence process. When the E-axis is started, the E-axis starts to move, and the E-axis value displayed on the CRT Change, when the value changes to 14, suddenly jump to 471, for this reason we think there is a problem in the feedback part, replace the position feedback board, the fault is eliminated.

Example 3 : Another CNC grinding machine, the E-axis dresser is out of control, the E-axis can return to the reference point, but when it is automatically trimmed or semi-automatic, the movement speed is extremely fast until it hits the limit switch. Observing the process of failure, it is found that the coordinate value displayed by the limit switch is much smaller than the actual value, which is definitely a problem of position feedback. However, replacing the feedback board and the encoder did not solve the problem. After careful study, it was found that the E-axis dresser is driven by the Z-axis. Generally, when the reference point is returned, the E-axis is on one side of the Z-axis, and when trimming, the E-axis dresser is brought to the middle by the Z-axis. To this end, we did this test, moving the E-axis dresser to the middle of the Z-axis, and then returning to the reference point. At this time, the return point also appeared out of control; for this reason, we concluded that the E-axis dresser may reciprocate frequently, resulting in The E-axis feedback cable is broken and the contact is poor. The school line confirmed our judgment, found breakpoints, welded and took anti-fold measures to get the machine back to work.

Third, external failure

Due to the increasing variability of modern CNC systems, the failure rate is getting lower and lower, and few failures occur. Most faults are non-system faults and are caused by external causes.

1. Modern numerical control equipment is a mechatronics product with complex structure, perfect protection measures and high degree of automation. Some faults are not caused by hardware damage, but by operation, adjustment, and improper handling. This type of failure occurs frequently at the beginning of the device's use, when operators and maintenance personnel are not particularly familiar with the equipment.

Example 1. A CNC milling machine, when it is just put into use, the rotary table often has the problem of non-rotation. After analyzing the working principle and processing of the machine tool, it is found that this problem is related to the indexing device. Only the indexing device is in The workbench can be rotated at the starting position.

Example 2 : Another CNC milling machine has a knife accident. After pressing the emergency stop button, the new knife is replaced, but the workbench does not rotate. Through the PLC ladder diagram analysis, it is found that the tool change process is not correct. The computer thinks that the tool change process is not At the end, no other operations can be performed. After the tool is changed again according to the correct procedure, the machine tool returns to normal.

Example 3 : There are several CNC machine tools. When they are just put into use, sometimes there is an unexpected situation. After the operator presses the emergency stop button, the system will be powered off and restarted. At this time, the machine does not return to the reference point and must be adjusted. Sometimes the shaft is manually moved to the non-interference zone. After learning the lesson, press the emergency stop button, change the operation mode to manual, release the emergency stop button, return the machine to the normal position, then operate or power off, there will be no problem.

2. Fault caused by external hardware damage

Such faults are common faults in CNC machine tools, and are generally caused by problems such as detection switches, hydraulic systems, pneumatic systems, electrical actuators, and mechanical devices. Some faults can generate an alarm, and by replying to the information, you can find the cause of the fault.

Example 1 , a CNC grinding machine, CNC system using Siemens SINUMERIK SYSTEM 3, fault alarm F31 "SPINDLE COOLANT CIRCUIT", indicating that there is a problem with the spindle cooling system, and checking the cooling system is no problem, consult the PLC ladder diagram, this fault is caused by The flow detection switch B9.6 detects it. Check this switch and find that the switch is damaged. Replace the new switch and the fault disappears.

Example 2 : A CNC quenching machine with Siemens SINUMERIK 810, a 6014 "FAULT LEVEL HARDENING LIQUID" machine can not work. The alarm message indicates that the quenching liquid level is not enough, the liquid level has been checked far beyond the minimum level, and the liquid level switch is detected. It is found that there is a problem with the liquid level switch, a new switch is replaced, and the fault is eliminated.

Some faults have alarm information, but they do not reflect the root cause of the fault. At this time, it is necessary to analyze according to the alarm information and the fault phenomenon.

Example 3 : For a CNC grinding machine, when the E-axis is returned to the reference point, the E-axis rotates but the reference point is not found, but moves until the limit switch is pressed, and the NC system displays the alarm a€?EAXIS AT MAX.TRAVELa€?. According to the analysis of the fault phenomenon, there may be a problem with the zero point switch. It is confirmed that the non-contact zero point switch is damaged, the new switch is replaced, and the fault is eliminated.

Example 4 : A special CNC milling machine fails during the batch processing of the parts. Every time it happens, the part has been processed, and the Z axis is not moved backward. At this time, the fault occurs, the machining program is interrupted, and the spindle stops. And the F97 alarm "SPINDLESPEED NOT OK STATION 2" is displayed, indicating that there is a problem with the spindle. There is no problem in checking the spindle system. Other problems can also cause the spindle to stop. So we use the off-machine programmer to monitor the running status of the PLC ladder and find out The tool hydraulic clamping pressure detecting switch F21.1 is momentarily disconnected in the event of a fault. Its disconnection indicates that the milling cutter has insufficient clamping force. For safety reasons, the PLC stops the spindle. After inspection, it was found that the hydraulic pressure was unstable, the hydraulic system was adjusted to make it stable, and the fault was eliminated.

There are still some faults that do not produce a fault alarm, but the action cannot be completed. At this time, the fault should be judged according to the maintenance experience, the working principle of the machine tool, and the operating state of the PLC.

Example 5 : A CNC machine tool fails once, the load door cannot be closed, automatic machining cannot be performed, and there is no fault display. The load gate is closed by the cylinder, and the closed load gate is realized by the PLC output Q2.0 control solenoid valve Y2.0. Check the PLC with the PC function of the NC system

The state of Q2.0 has a state of 1, but the solenoid valve does not have power. The original PLC output Q2.0 controls the solenoid valve Y2.0 through the intermediate relay. The intermediate relay is damaged and causes this fault. The new relay is replaced and the fault is eliminated.

Example 6 : A CNC machine tool, the workbench does not rotate, and the NC system does not display a fault alarm. According to the action principle of the workbench, the first step of the worktable rotation should float the workbench pneumatically. Use the off-machine programmer to track the dynamic change of the PLC ladder diagram. It is found that the PLC signal is not sent out. Continue to view according to this clue. Finally, It is found that the second and third station indexing head starting position detecting switches I9.7 and I10.6 are not synchronized, resulting in the table not rotating. It is further confirmed that the three-station indexing head generates a mechanical misalignment, and the mechanical device is adjusted to synchronize with the two stations, so that the fault is eliminated.

Finding the problem is the first step in solving the problem and is the most important step . Especially for the external faults of CNC machine tools, sometimes the diagnosis process is more complicated. Once the problem is found, it is easier to solve. For the diagnosis of external faults, we have summarized two experiences. First , we should be proficient in the working principle and sequence of the machine tools. Secondly, it is necessary to skillfully use the PLC ladder diagram provided by the factory, use the status display function of the NC system or use the external programmer to monitor the operating status of the PLC. According to the chain lock relationship of the ladder diagram, determine the fault point, as long as the above two points are achieved, The external faults of general CNC machine tools will be eliminated in time.


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